1. Field of the Invention
The present invention relates to an apparatus for producing a silicon single crystal grown by the Czochralski method also known as pulling method.
2. Description of the Prior Art and Related Art
Conventionally, in the production of a silicon single crystal grown by the Czochralski method (hereinafter referred to as "CZ" method), a silicon single crystal producing apparatus such as shown here in FIG. 2 of the accompanying drawings is used. The apparatus includes a main chamber 6 composed of a side wall 6a and an upper wall 7 having at its central portion a neck 4. Hot melt parts, which are used to heat the raw material to melt the same and keep the raw material in the molten or liquid state, are disposed at the central portion of the main chamber 6. The hot melt parts are composed of a quartz crucible 12 protected by a graphite crucible 11, and a heater 16 and a thermal shield or insulator 18 that are disposed around the crucibles 11, 12. The apparatus further includes a gate valve 8 disposed above the neck 4 of the main chamber 6, a pull chamber (not shown) and a crystal lift device including a lift mechanism (neither shown) that are disposed above the gate valve 8. An exhaust port 19 is connected with a vacuum pump (not shown) for controlling the pressure of an inert gas within the main chamber 6 and discharging the inert gas from the main chamber 6.
In operation, in an inert gas atmosphere held under reduced pressure, a seed crystal 26 fixed to the front end of a pull shaft 24, such as a wire, is dipped into a silicon melt 14 in the quartz crucible 12 while the quartz crucible 12 is rotating about its own axis. The seed crystal 26 while being rotated is then pulled up from the silicon melt 14 so that a silicon single crystal 28 is produced.
The single crystal produced by the CZ method is used to fabricate silicon wafers (hereinafter referred to as "CZ wafers"). It has been known that the CZ wafers have a dielectric breakdown strength of oxide film (SiO.sub.2) which is considerably lower than that of those wafers fabricated from a single crystal produced by the floating zone method (FZ method), or that of epitaxial wafers each composed of a thin silicon film epitaxially grown on a CZ wafer. However, due to various advantageous features, the CZ wafers are widely used as a material for fabricating semiconductor devices.
More particularly, in recent years, with the advance of high integration of semiconductor circuits and accompanying microminiaturization of devices, the thickness of a dielectric oxide film at the gate electrode of a MOS-LSI chip has been reduced. However, even in the case of a thin dielectric oxide film, high qualities, such as a high dielectric breakdown strength and a small leakage current during operation of a device, are required. The oxide-film dielectric strength can be deteriorated by, for example, a defect induced by a thermal history during the growth of a silicon single crystal. It has been known that the dielectric breakdown strength characteristic is improved when the crystal growth rate is slowed down to change the thermal history of a silicon single crystal (see, Japanese Patent Laid-open Publication No. 2-267195, for example). The known method, however, requires a slowing down of the crystal growth rate in order to obtain a silicon single crystal with a high dielectric breakdown strength (of oxide film). The productivity of the single crystal growth process is reduced correspondingly.
In order to overcome the foregoing problem of the prior art, the behavior of heat treatments toward the CZ wafers was investigated by the present inventors. Through the investigation, the present inventors found that the dielectric breakdown strength characteristic can be improved by a high-temperature heat treatment conducted at temperatures above 1150.degree. C. Based on that founding, the present inventors proposed a method which is capable of stably producing a silicon single crystal with high productivity by heat-treating a growing single crystal rod at temperatures above 1150.degree. C. for a certain period of time within a growing furnace, and an apparatus suitable for reducing the method into practice (see, Japanese Patent Laid-open Publication No. 5-70283).
Through a further investigation, the present inventors found the proposed apparatus not to be fully satisfactory in that due to the flat and highly emissive top wall of a main chamber such as shown in FIG. 2, the heat generated from a growing single crystal is mostly scattered and lost.